1. Technical Field
The present invention relates to a deformable mirror device appropriate for use in retinal cameras, heads-up displays, astronomical telescopes, laser irradiation devices and so on. The present invention also relates to a device for observing the retina of an eye that casts light beam from a photographing light source to an eye to be examined and records the image of the light beam reflected from the retina of the eye as a retinal image, for diagnosing the retina of the eye.
2. Related Art
The device such as a camera for observing the retina of an eye is used by ophthalmologists and ophthalmic opticians to photograph the image of the retina of an eye for inspecting the state of the retina, hemorrhage on the retina of the eye, and so on. Incidentally, the human eyes optical system is composed of the cornea, the lens, the vitreous body and others with, unlike an ideal optical system used as a basis of the geometrical optics, some deformation. In particular in the clinical field of ophthalmology, the image of the retina of the eye is required to be clear and of little aberration because the extent of difference of the examined eye from a normal eye is used as diagnosis information. However, because the optical system for the human eyes constituting the photographing device is not ideal, in some cases sufficient resolution cannot be achieved. Therefore, to compensate for the deformation of the wavefront of the optical system for the human eyes, the deformable mirrors using the piezoelectric effect have been in use.
However, the conventional deformable mirrors using the piezoelectric element require a high voltage applied to the piezoelectric element and needs to use, as an electronic control circuit, a piezoelectric element with a high dielectric strength that is expensive. Therefore, commercially available retinal cameras employ deformable mirrors using electrostatic attraction that can be actuated with a lower voltage in comparison with the piezoelectric type.
However, the electrostatic type of deformable mirrors is low in natural frequency (for example, about 10 Hz) and the response speed is about 100 milliseconds in time constant. Thus, the electrostatic type of deformable mirrors, because of its slow response in deformation, has had a problem of difficulty in performing real time processing to the extent required in the ophthalmic examination (for example, a response speed of about 30 milliseconds in time constant).
Another problem with the deformable mirrors is that its shape is determined with the limitation of the specifications of the instruments to which it is incorporated, and its thickness, size, and material cannot be changed freely. Another problem with the deformable mirrors is that its dynamic range for the deformation amount is determined with the specifications of the instruments to which it is incorporated, and the distance between electrodes of the electrostatic type of deformable mirrors is also determined accordingly. Therefore, the response characteristic of the electrostatic type of deformable mirrors to the applied voltage is inevitably determined. Thus, it is still another problem that no measures can be taken in the structural design to shorten the response time of the deformable mirrors, such measures as employing hard materials for the deformable mirrors, increasing the thickness of deformable member, and reducing the size of the deformable member.
The present invention is to solve the above problems. An object of the present invention is to provide a deformable mirror device with which response characteristic is improved by devising the mode of control even if the time constant of the deformable mirror is large in comparison with the response speed required in applications such as images of the retina of an eye. Another object of the present invention is to provide a device for observing the retina of an eye with which a real time processing can be performed to the extent required in ophthalmic examinations.